Ventilators in the operating theatre

You are ventilating a patient in theatre using a simple bag-in-bottle ventilator connected

to the common gas outlet. You are using a fresh gas flow of 1 L/min, a circle

breathing system and volume control ventilation mode. Using spirometry connected to

your anaesthetic machine you note a tidal volume of 500 mL, a respiratory rate of

12 breaths per minute and an I:E ratio of 1:2. You need to rapidly affect a change in

circuit concentration of volatile anaesthetic agent. You increase your fresh gas flow to

4 L/min. You leave all the ventilator settings unchanged. One minute later, what

delivered minute volume would you expect the spirometry to be registering?

a) 5 L/min

b) 6 L/min

c) 7 L/min

d) 8 L/min

e) 9 L/min

Answer: c

Explanation

This question regards gas flow coupling in bag-in-bottle ventilators. As these are still

among the commonest ventilators used on anaesthetic machines it is a phenomenon

that can be witnessed in our daily work. Fresh gas flow is delivered into the bellows of

the ventilator, which are themselves contained within a glass chamber (so the bellows

can be visualised to be filling and moving appropriately). A separate gas source

provides an intermittent pneumatic pressure into the chamber, which when applied

to the bellows causes them to collapse (to a controllable extent), ejecting a proportion of

their contents into the breathing system.Aspill valve in the chamber then opens during

expiration allowing the pressure to drop and the bellows to refill from the fresh gas

flow. During inspiration the volume delivered to the breathing circuit will be that

volume set on the ventilator (and consequently delivered to the chamber) PLUS whatever

volume of fresh gas flow is delivered to the bellows and circuit during this

inspiratory phase. This is because during inspiration the spill valve in the chamber is

closed to allow the necessary pressurisation of the chamber such that the fresh gas flow,

which is being delivered continuously, must enter the bellows and breathing system

thus augmenting the volume delivered on that breath. In the example here, the fresh

gas flow has been increased by 3 L/min. With an I:E ratio of 1:2, one third of every

minute is spent in inspiration, during which time the fresh gas flow is added to the set

volume. One third of 3 L is 1 L, so our measured minute volume will be seen to increase

by 1 L/min. Prior to the increase it was 500mL × 12 = 6 L/min, so now it is 7 L/min.

Incidentally, during the expiratory phase once the volume to the bellows is restored

any excess fresh gas flow will be vented to the scavenging (to avoid pressurising the

system), which is neither economical nor environmentally sound.